343 is trying to keep the number of playlists at a manageable number so it doesn't become the mess that the Halo: Reach playlist became. As a result, they are rotating out playlists that aren't necessarily popular.

I'm not defending the practice, just pointing out their reasoning.

There's no skill ranking in Halo 4 yet but it is supposed to be coming in a future title update. Maybe they'll add a ranked playlist then? <shrug>